1. Field of the Invention
The invention relates to an engine control system for controlling an engine to operate in different engine operation modes according to operating conditions.
2. Description of the Related Art
Typically, in order to secure engine output torque in an engine operating zone of higher engine loads and provide an improved fuel consumption in an engine operating zone of lower engine loads, it has been well known in the art to operate an engine in a plurality of engine operation modes different fuel injection form (injection timing, air-to-fuel ratio, etc.) according to operating conditions.
There have been known engine control systems which control engines of a type of injecting fuel directly into combustion chambers to operate in different combustion modies, namely a stratified charge combustion mode in which fuel injection is made in a compression stroke and stratified charge combustion is performed with a fuel mixture at a significantly higher air-to-fuel ratio, a homogeneous charge combustion mode in which fuel injection is made in an intake stroke and homogeneous charge combustion is performed with a fuel mixture it a relatively high air-to-fuel ratio, and a homogeneous charge combustion mode in which fuel injection is made in an intake stroke and homogeneous charge combustion is performed with a fuel mixture at a stoichiometric air-to-fuel ratio. Such an engine control system is known from, for example, Japanese Unexamined Patent Publication No. 8-312402. This engine control system determines a slow response group of control parameters, such as the amount of exhaust gas recirculation and a bypass intake air flow rate, on the basis of an engine load calculated based on an engine operating condition and determines a fast response group of control parameters, such as the amount of fuel injection, a fuel injection timing and an ignition timing, calculated based on a parameter related to the engine load after a correction in relation to a response delay, so as thereby to adjust control timing between the fast and slow response control parameters.
The engine control system described in the Japanese Unexamined Patent Publication No. 8-312402 is hard to have control timing for the control parameters completely adjusted. In particular, in the case where intake air flow control means such as a swirl control valve operative to regulate a swirl of intake air is employed to change an intake air flow, there is a problem that engine output torque fluctuates due to temporary aggravation of combustion during a shift between operation modes. Specifically, operation of the intake air flow control means, e.g. opening of a swirl control valve, changes according to the engine operation modes and, in consequence, during an operation mode shift, the intake air flow control means is accompanied by a delay in attaining an operated condition suitable for an engine operation mode into which the operation mode shift is made. An intake air flow condition is made contrary to a suitable condition for the engine operation mode into which the operation mode shift has been made due to the delay, which causes temporary aggravation of combustion and a change in engine output torque. Further, in the case where the engine is equipped with an exhaust gas recirculation system, exhaust gas recirculation control means such as an EGR valve is accompanied by a delay in attaining an operated condition suitable for an engine operation mode into which the operation mode shift is made, which possibly causes temporary aggravation of combustion and a change in engine output torque.